H-2 IN INTERSTELLAR AND EXTRAGALACTIC ICES - INFRARED CHARACTERISTICS, ULTRAVIOLET PRODUCTION, AND IMPLICATIONS

H-2 is the most abundant molecule in the universe. We demonstrate that this molecule may be an important component of interstellar and possi bly intergalactic ices, both because it can be formed in situ, within the ices, and because gas phase H-2 can freeze out onto dust grains in some astrophysical environments. The condensation-sublimation and inf rared spectral properties of ices containing H-2 are presented. We sho w that solid H-2 in H2O-rich ices can be detected by an infrared absor ption band at 4137 cm-1 (2.417 mum). The surface binding energy of H-2 to H2O ice was measured to be DELTAH(s)/k = 555 +/- 35 K. Surface bin ding energies can be used to calculate the residence times of H-2 on g rain surfaces as a function of temperature. Some of the implications o f these results are considered.